The broad, long-term objective of this project is to understand the ionic mechanisms for neuromodulation of the locomotor network in the vertebrate spinal cord. The significance of this objective is that 1) neuromodulation is an important mechanism for the control of neuronal networks, and 2) understanding how neuromodulators alter network behavior on the basis of their actions on individual ionic currents provides basic insights into network function. Such understanding is fundamental to future work aimed at developing effective therapeutic measures for treating spinal cord dysfunction resulting from injury, disease, and aging. This project will focus on two neuromodulators, serotonin and dopamine, that ar known to exist in the lamprey spinal cord and that powerfully alter the central pattern generator for locomotion. The lamprey is a primitive vertebrate fish that has become a model system for investigating vertebrate locomotor pattern generation. The lamprey spinal cord exhibits the neuronal correlate of swimming in vitro and much is known about the properties and synaptic interactions of the spinal neurons involved in this locomotor activity. Both serotonin and dopamine reduce the late after hyperpolarization following the action potential in neurons, and thus alters their firing rates. However, based on preliminary data it is hypothesized that 1) these two neuromodulators act on different ionic currents to reduce the late after hyperpolarization, and 2) they have different effects on different classes of spinal neurons. The specific aims of this project are to test these two hypotheses by making whole-cell patch clamp recordings of isolated lamprey neurons that have been acutely dissociated from adult spinal cord. The cells will be characterized by making quantitative measurements of their ionic currents, and by their size, morphology, and their axonal projection. The latter will be based on the presence of a fluorescent tracer from prior retrograde labeling. The effects of dopamine and serotonin on isolated ionic currents will be examined in a variety of neuron classes. New findings from the experiments will be confirmed using conventional intracellular microelectrodes on the in vitro preparation of the lamprey spinal cord. The experiments proposed here will advance our understanding of the mechanisms of vertebrate locomotion by 1) providing quantitative measurements of the ionic currents of neurons in a model system, and 2) determining the sites of action of two potent neuromodulators of the locomotor network.